Total organic carbon measuring instrument

ABSTRACT

A total organic carbon measuring instrument including a measuring unit composed of, integrated together, organic substance oxidation part ( 24 ) and carbon dioxide separation part ( 20 ) and conductivity measuring part ( 34 ), control unit ( 40 ) and data processing unit ( 41 ). In order to enhance the accuracy of conductivity measurement, the control unit ( 40 ) is constructed so as to stop feeding of a sample water at the time of oxidation of organic substance and carry out feeding of the sample water at the time of sample water moving to the organic substance oxidation decomposition part ( 20 ) and carbon dioxide separation part ( 24 ). The data processing unit ( 41 ) is constructed so as to measure the total organic carbon concentration on the basis of conductivity at the time of arriving of sample water irradiated with ultraviolet rays of which relative intensity is a given value or higher at the carbon dioxide separation part ( 24 ).

TECHNICAL FIELD

The present invention relates to a total organic carbon measuringinstrument (hereinafter, also referred to as a “TOC measuringinstrument”) for measuring the total organic carbon (TOC) content of awater sample. More particularly, the present invention relates to a μTAS(Micro Total Analysis System) obtained by integrating some functionsnecessary for measuring the TOC content of low-impurity water called“pure water” or “ultra-pure water” such as separation of organicsubstances by a carbon dioxide separation part and measurement ofconductivity by a conductivity measuring part.

BACKGROUND ART

The amount of organic substances contained in low-impurity water such aswater for pharmaceutical production, process water for semiconductormanufacturing, cooling water, boiler water, or tap water is monitored bymeasuring the TOC content of a water sample.

Examples of a total organic carbon measuring method include a combustionoxidation method in which organic substances are oxidized by combustionin a high-temperature furnace and a wet oxidation method in whichorganic substances are chemically oxidized using UV light and oxidants.In the case of TOC measurements of pure water and ultra-pure waterrequiring high sensitivity, the latter, that is, a wet oxidation methodis generally used.

As a method for measuring TOC by wet oxidation, there is a methodincluding the steps of: converting organic substances contained in awater sample to carbon dioxide by an oxidation reactor; transferring thecarbon dioxide into measurement water through a gas-permeable membrane;and feeding the measurement water containing the carbon dioxidetransferred from the water sample to a conductivity measuring unit tomeasure the conductivity of the measurement water to detect theconcentration of carbon dioxide (see Patent Documents 1 and 2).

Further, as conductivity measurement of carbon dioxide, there is also amethod of measuring TOC of an organic compound in which at least twoelectrodes are provided at positions before and after oxidation, and adifference in the conductivity of the water sample between before andafter oxidation is detected (see Patent Document 3).

As a method for measuring the TOC content of low-impurity water such asprocess water for semiconductor manufacturing or water forpharmaceutical production, there is a method in which organic substancescontained in a water sample are decomposed by UV light to carbondioxide, the carbon dioxide is transferred into measurement waterthrough a carbon dioxide separation part, and the conductivity of themeasurement water is measured. Such a method is known as a methodcapable of measuring the TOC content of low-impurity water with highaccuracy using a relatively compact instrument.

The present inventors have already developed a total organic carbonmeasuring instrument obtained by integrating some devices using amicrofabrication technique to suggest a reduction in the volume of waterto be measured as compared to conventional measuring instruments (seePatent Document 4). This TOC measuring instrument is intended to achieveboth a reduction in the consumption of a water sample and a reduction inthe influence of elution of piping materials and/or the influence ofcarbon dioxide to be transferred.

Patent Document 1: Japanese Patent No. 2510368

Patent Document 2: Japanese Patent Application Laid-open No. 2006-90732

Patent Document 3: Japanese Patent Application Laid-open No. 2001-281189

Patent Document 4: Japanese Patent Application Laid-open No. 2006-300633

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In the case of using such a total organic carbon measuring instrumentobtained by integrating some devices, it is necessary to stop thefeeding of a water sample during irradiation with UV light tosufficiently oxidize organic substances. However, since the intensitydistribution of UV light emitted from a UV lamp is not uniform, theorganic substances contained in the water sample are not uniformlyoxidized in a flow channel of an organic substance oxidation part. Inthis case, although carbon dioxide is generated by oxidation of theorganic substances, the concentration of carbon dioxide in the watersample is low in part of the flow channel not sufficiently irradiatedwith UV light, and therefore, there is a problem that the concentrationof TOC of the water sample becomes lower than its true value because theconductivity of measurement water is not sufficiently increased evenafter the water sample arrives at the carbon dioxide separation part.

Therefore, it is an object of the present invention to provide a totalorganic carbon measuring instrument capable of enhancing the accuracy ofconductivity measurement.

Means for Solving the Problem

The present invention is directed to a total organic carbon measuringinstrument including a measuring unit, a control unit for controllingthe operation of liquid delivery in the measuring unit, and a dataprocessing unit for determining the concentration of total organiccarbon from the conductivity of measurement water. The measuring unithas an organic substance oxidation part for oxidizing organic substancescontained in a supplied water sample to carbon dioxide by irradiationwith UV light, a carbon dioxide separation part having a water samplechannel through which a water sample transferred from the organicsubstance oxidation part flows and a measurement water channel throughwhich measurement water constituted from deionized water flows, thecarbon dioxide separation part being formed by integrating the watersample channel and the measurement water channel being in contact witheach other gas-permeably and laminated in this order from top to bottom,and a conductivity measuring part for measuring a conductivity ofmeasurement water transferred from the carbon dioxide separation part.The measuring unit is formed by laminating the organic substanceoxidation part, the carbon dioxide separation part, and the conductivitymeasuring part in this order from top to bottom.

The control unit stops feeding the water sample during oxidation oforganic substances and performs feeding the water sample when the watersample is moved to the carbon dioxide separation part through theorganic substance oxidation part. The data processing unit determinesthe concentration of total organic carbon based on conductivity measuredwhen the water sample irradiated with UV light of which relativeintensity is equal to or higher than a certain value is flowing throughthe carbon dioxide separation unit.

When organic substances contained in a water sample are decomposed by UVlight, there is a case where gas components other than carbon dioxideare generated from compounds contained in the water sample and havingelements other than carbon such as nitrogen compounds, and are thentransferred into measurement water together with carbon dioxide, therebyadversely affecting conductivity measurement.

The carbon dioxide separation part may have, between the water samplechannel and the measurement water channel, an intermediate water channelin which intermediate water having a pH higher than that of a watersample flowing through the water sample channel but within a neutralrange is present. In this case, the water sample channel and theintermediate water channel are in contact with each other with agas-permeable membrane being interposed therebetween, and theintermediate water channel and the measurement water channel are incontact with each other with a gas-permeable membrane being interposedtherebetween. Further the water sample channel, the intermediate waterchannel, and the measurement water channel are laminated in this orderfrom top to bottom and integrated.

Generally, in the case of determining the TOC content of a water samplebased on the conductivity of measurement water measured using a TOCmeasuring instrument having a carbon dioxide separation part, the watersample is made strongly acidic by adding an acid to the water sample inorder to remove original dissolved carbon dioxide, promote the transferof gas components into the water sample, and stabilize measurement. Whena water sample containing nitrogen compounds such as urea and the likeis irradiated with UV light under strongly acidic conditions, thenitrogen compounds are oxidatively decomposed so that nitric acid andnitrous acid are generated.

As shown in FIG. 6, the ratio between nitrous acid and nitrite ionpresent in water varies depending on pH. More specifically, nitrous acidis present as a gas component under acidic conditions but present asnitrite ion under neutral to alkaline conditions.

In the case of a conventional TOC measuring instrument having a carbondioxide separation part in which a gas-permeable membrane is interposedbetween an acidic water sample and neutral measurement water, nitrousacid generated in the water sample is transferred through thegas-permeable membrane into the measurement water and is then present asnitrite ion. As a result, nitrous acid causes positive interference inconductivity measurement, that is, the conductivity of the measurementwater becomes high.

However, in the case of the TOC measuring instrument according to thepresent invention, by providing, between the water sample channel andthe measurement water channel, an intermediate water channel in whichthe intermediate water having a pH higher than that of a water sampleflowing through the water sample channel but within a neutral range ispresent, it is possible to suppress the transfer of nitrous acid intomeasurement water. This is due to the following reasons. Nitrous acidgenerated in a water sample is transferred into the intermediate waterchannel through the gas-permeable membrane. However, the ratio ofnitrous acid present as a gas component in the intermediate water isdecreased by maintaining the pH of the intermediate water aroundneutral, and then nitrous acid in the intermediate water is present asnitrite ion which cannot pass through the gas-permeable membrane.

In a case where the intermediate water and the measurement water areboth, for example, deionized water, the pH of each of the intermediatewater and the measurement water is maintained at 5 to 7 by dissolvedcarbonic acid. Under such conditions, the carbonic acid component ismostly present as a gas component, but nitrous acid is mostly present asan ion component. The rate of gas transfer from the intermediate waterto the measurement water is determined by the difference in gasconcentration between them, and therefore, the transfer rate of nitrousacid is lower than that of carbonic acid because nitrous acid is mostlypresent as ion. By appropriately designing the thickness of thegas-permeable membrane interposed between the intermediate water and themeasurement water, the area of contact between the gas-permeablemembrane and the intermediate water and the area of contact between thegas-permeable membrane and the measurement water based on the differencein transfer rate between carbonic acid and nitrous acid, it is possibleto reduce the influence of nitrous acid on carbon dioxide.

By providing such an intermediate water channel, it is possible toachieve both a high transfer rate of carbon dioxide and a reduction inthe influence of interfering substances. Nitrous acid is exemplified asan interfering substance, but the influence of other interferingsubstances can be diminished as long as they are present in a gaseousstate under acidic conditions but are ionized under neutral to alkalineconditions.

Further, even when the TOC measuring instrument according to the presentinvention has a multiple structure due to providing the intermediatewater channel so that it becomes difficult to keep the flow rate ratioamong a water sample, intermediate water, and measurement water, and thetiming of liquid delivery constant, the TOC content of a water samplecan be accurately determined because the water sample is sufficientlyoxidized and is then allowed to flow through the carbon dioxideseparation part.

An example of the organic substance oxidation part of the total organiccarbon measuring instrument according to the present invention includesone having a flow channel through which a water sample flows and a UVlight incident portion for allowing the water sample flowing through theflow channel to be irradiated with UV light. Such an organic substanceoxidation part using UV light can be reduced in size and easilyintegrated with other parts because it does not need a heating portionor a pressuring portion. The flow channel of the organic substanceoxidation part through which the water sample flows may meander in theUV light incident portion to have an increased flow channel length. Thismakes it possible to increase the time for UV light irradiation toenhance oxidation efficiency.

In order to increase the time during when the water sample remains incontact with the intermediate water in the carbon dioxide separationpart to enhance the efficiency of gas transfer from the water sampleinto the intermediate water, part of the water sample channel being incontact with the gas-permeable membrane may meander to have an increasedflow channel length.

In order to increase the time when the measurement water remains incontact with the intermediate water in the carbon dioxide separationpart to enhance the efficiency of gas transfer from the intermediatewater into the measurement water, part of the measurement water channelbeing in contact with the gas-permeable membrane may meander to have anincreased flow channel length.

Further, in order to increase the retention time of the intermediatewater in the intermediate water channel, part of the intermediate waterchannel being in contact with the gas-permeable membranes may meander tohave an increased flow channel length.

It is important for the intermediate water to set its pH. Examples ofintermediate water include, in addition to pure water or deionizedwater, a buffer solution having a pH within a neutral range.

Effects of the Invention

According to the present invention, as described above, since thecontrol unit stops feeding the water sample during oxidation of organicsubstances and performs feeding the water sample when the water sampleis moved to the carbon dioxide separation part through the organicsubstance oxidation part, and the data processing unit determines theconcentration of total organic carbon based on conductivity measuredwhen a water sample irradiated with UV light of which relative intensityis equal to or higher than a certain value is flowing through the carbondioxide separation unit, it is possible to accurately measure the TOCvalue of the water sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of one embodiment of a totalorganic carbon measuring instrument according to the present invention;

FIG. 2 is a schematic sectional view of another example of a carbondioxide separation part;

FIG. 3A is a plan view showing an example of an oxidation channel havingan unoptimized flow channel pattern;

FIG. 3B is a plan view showing an example of an oxidation channel havingan optimized flow channel pattern;

FIG. 4A is a graph showing the result of experiment performed using theflow channel pattern shown in FIG. 3A;

FIG. 4B is a graph showing the result of experiment performed using theflow channel pattern shown in FIG. 3B;

FIG. 5 is a schematic sectional view of another example of the carbondioxide separation part; and

FIG. 6 is a graph showing molecular component ratio-versus-pH curves ofnitrous acid and carbon dioxide.

DESCRIPTION OF THE REFERENCE NUMERALS

2 water sample channel

4 intermediate water channel

6 measurement water channel

8, 10 gas-permeable membrane

20 carbon dioxide separation part

24 organic substance oxidation part

34 conductivity measuring part

40 control unit

41 data processing unit

42 liquid feeding device

64 water sample inlet

66 water sample outlet

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, an embodiment of the present invention will be described.

FIG. 1 is a schematic sectional view of one example of a total organiccarbon measuring instrument according to the present invention. Thetotal organic carbon measuring instrument includes a measuring unit, aliquid feeding device 42 for feeding a water sample to the measuringunit, a control unit 40 for controlling the liquid feeding device 42,and a data processing unit 41 for determining TOC based on conductivity.The measuring unit is obtained by integrating an organic substanceoxidation part 24, a carbon dioxide separation part 20, and aconductivity measuring part 34. It is to be noted that when it isnecessary to differentiate between the top surface and the back surfaceof each substrate, the upper surface and the lower surface thereof shownin FIG. 1 will be referred to as “top surface” and “back surface”,respectively.

The organic oxidation part 24 is constituted from a substrate 60 onwhich UV light is incident and a substrate 62 bonded to the substrate60. As the substrate 60, a quartz substrate allowing UV light to passthrough it is used to decompose organic substances by UV light emittedfrom above. The substrate 60 has a UV light incident portion on which UVlight is incident. The substrate 60 also has a through hole 64 servingas a water sample inlet and a through hole 66 serving as a water sampleoutlet. The substrate 62 is also formed from a quartz substrate. Thesubstrate 62 has an oxidation channel 68 provided in the top surfacethereof so that one end of the oxidation channel 68 is located at aposition corresponding to the water sample inlet 64. The substrate 62has a water sample channel 2 provided in the back surface thereof sothat one end of the water sample channel 2 is located at a positioncorresponding to the water sample outlet 66. Further, the substrate 62has a through hole 70 through which the other end of the oxidationchannel 68 and the other end of the water sample channel 2 are connectedto each other and a through hole 72 through which the one end of thewater sample channel 2 and the water sample outlet 66 are connected toeach other. A light-blocking metal film 33 is provided on the backsurface of the substrate 62, that is, on one surface of the substrate 62opposite to the other surface thereof being in contact with thesubstrate 60. The light-blocking metal film 33 defines a regionirradiated with UV light. An example of the light-blocking metal film 33includes a Pt/Ti film (which is obtained by forming a platinum film on atitanium film provided as a bonding layer) having a thickness of 0.05 μmor more.

The sizes of the oxidation channel 68 and the water sample channel 2 arenot particularly limited. For example, the oxidation channel 68 and thewater sample channel 2 may be formed using a processing technique suchas wet etching or dry etching so as to have a width of about 1 mm, adepth of about 0.2 mm and a length of about 200 mm. The through holes64, 66, and 70 can be formed using a processing technique such assandblasting. The substrate 60 and the substrate 62 can be bondedtogether using hydrogen fluoride.

The conductivity measuring part 34 is formed by bonding an electrodepattern 76 formed from a Pt/Ti film on a quartz substrate 74 to the backsurface of a quartz substrate 80 with a film 78, part of which has beenremoved to form a flow channel pattern, being interposed therebetween.

Examples of the film 78 include an adhesive fluorine resin film (e.g., a100 μm-thick film made of Neoflon EFEP (“Neoflon” is a trademark ofDaikin Industries, Ltd.)) and a PDMS (polydimethylsiloxane) film (e.g.,a 100 μm-thick film made of Sylgard 184 (“Sylgard” is a trademark of DOWCORNING)). On the electrode pattern 76, a flow channel for allowingmeasurement water to flow through it is provided by the film 78.

The electrode pattern 76 can be formed by patterning a sputtered Pt/Tifilm by photolithography and etching used in the fields of semiconductormanufacturing and microfabrication. However, a method for forming theelectrode pattern 76 is not particularly limited. Further, a film forforming a flow channel on the electrode pattern 76 is not limited to aNeoflon film or a PDMS film. For example, an adhesive organic film or athin film coated with an adhesive may be used to form a flow channel onthe electrode pattern 76. Therefore, a method for forming a flow channelon the electrode pattern 76 is not limited to a method using a Neoflonfilm or a PDMS film.

The quartz substrate 80 has a measurement water channel 6 formed in thetop surface thereof. The quartz substrate 80 has a measurement waterbranch channel 82 connected to one end of the measurement water channel6 and a through hole 84 for connecting the other end of the measurementwater channel 6 to the flow channel provided on the electrode pattern 76of the conductivity measuring part 34. Further, the quartz substrate 80has a through hole 86 serving as an intermediate water branch channelfor guiding intermediate water and a through hole 88 serving as anintermediate water outlet for discharging intermediate water. Thethickness of the quartz substrate 80 is not particularly limited. Forexample, the quartz substrate 80 having a thickness of 1 mm is used.

The quartz substrate 74 has a through hole 90 serving as an ion exchangewater inlet for supplying ion exchange water as deionized water and athrough hole 92 serving as an ion exchange water outlet for dischargingexcess ion exchange water. The ion exchange water inlet 90 is connectedto the measurement water branch channel 82, the intermediate waterbranch channel 86, and the ion exchange water outlet 92 through the flowchannel formed by the PDMS film 78 interposed between the substrates 74and 80.

The quartz substrate 74 has a through hole 94 serving as a measurementwater outlet for discharging measurement water from the flow channelprovided on the electrode pattern 76 of the conductivity measuring part34 after conductivity detection and a through hole 96 connected to thethrough hole 88, which is provided as an intermediate water outlet inthe quartz substrate 80, to serve as an intermediate water outlet fordischarging intermediate water.

The carbon dioxide separation part 20 is provided by bonding togetherthe back surface of the substrate 62 constituting the organic substanceoxidation part 24 and the top surface of the substrate 80 constitutingthe conductivity measuring part 34 with two gas-permeable membranes 8and 10 being interposed therebetween. The gas-permeable membranes 8 and10 constitute the carbon dioxide separation part 20.

Further, a PDMS film 98 is interposed between the gas-permeablemembranes 8 and 10 to create a clearance corresponding to the thicknessof the PDMS film 98. The PDMS film 98 has a pattern corresponding to anintermediate water channel 4. The intermediate water channel 4 is formedso that one end thereof is connected to the intermediate water branchchannel 86 provided in the quartz substrate 80 to guide intermediatewater, and the other end thereof is connected to the through hole 88serving as an intermediate water outlet.

Intermediate water flowing through the intermediate water channel 4 hasa pH higher than that of a water sample but within a neutral range. Atleast part of the intermediate water channel 4 is parallel with thewater sample channel 2 and the measurement water channel 6 so as to comeinto contact with the water sample channel 2 and the measurement waterchannel 6 with the gas-permeable membranes 8 and 10.

The interface between the gas-permeable membrane 8 and the substrate 62is sealed with a film such as a PDMS film because the water samplechannel 2 is provided between the gas-permeable membrane 8 and thesubstrate 62. Also, the interface between the gas-permeable membrane 10and the substrate 80 is sealed with a film such as a PDMS film becausethe measurement water channel 6 is provided between the gas-permeablemembrane 10 and the substrate 80.

The gas-permeable membranes 8 and 10 are not particularly limited aslong as they do not have carbon dioxide selectivity. Examples of suchgas-permeable membranes 8 and 10 include porous fluorine resin membranes(e.g., a 30 μm-thick Poreflon membrane manufactured by Sumitomo ElectricFine Polymer, Inc.).

In the case of the total organic carbon measuring instrument accordingto this embodiment, a water sample is introduced through the watersample inlet 64 provided in the substrate 60, flows through theoxidation channel 68 and the water sample channel 2, and is thendischarged through the water sample outlet 66. More specifically, thewater sample introduced into the organic substance oxidation part 24through the water sample inlet 64 is oxidized by irradiation with UVlight, and is then brought into contact with intermediate waterseparated by the gas-permeable membrane 8 of the carbon dioxideseparation part 20 so that gas components such as carbon dioxide aretransferred into the intermediate water.

Ion exchange water is produced by an external unit and is thenintroduced through the ion exchange water inlet 90. Most of the ionexchange water introduced through the ion exchange water inlet 90 isdirectly discharged through the ion exchange water outlet 92, but only anecessary amount of the ion exchange water is supplied to themeasurement water channel 6 and the intermediate water channel 4 throughthe measurement water branch channel 82 and the intermediate waterbranch channel 86, respectively.

The intermediate water channel 4 is in contact with both thegas-permeable membrane 8 to be brought into contact with the watersample and the gas-permeable membrane 10 to be brought into contact withthe measurement water, and therefore, gas components transferred fromthe water sample into the intermediate water are distributed to themeasurement water while keeping equilibriums with their respective ionsgenerated in the intermediate water, and then the intermediate water isdischarged to the outside through the intermediate water outlets 88 and96. On the other hand, the measurement water flowing through themeasurement water channel 6 receives the gas components, flows throughthe flow channel provided on the electrode pattern 76, and is thendischarged through the measurement water outlet 94.

The feeding of the water sample is controlled by operating the liquidfeeding device 42 and the control unit 40. More specifically, thefeeding of the water sample is stopped during oxidation of organicsubstances, and is performed only when the water sample is moved to thecarbon dioxide separation part 20 through the organic substanceoxidation part. The data processing unit 41 determines the concentrationof total organic carbon based on conductivity measured when the watersample irradiated with UV light of which relative intensity is equal toor higher than a certain value is flowing through the carbon dioxideseparation part 20.

In the case of the total organic carbon measuring instrument accordingto this embodiment, for example, a pen-type low-pressure mercury lamp(L937-02) manufactured by Hamamatsu Photonics K.K. can be used as a UVlamp emitting UV light with which the water sample is irradiated. Thelight intensity distribution of this lamp is shown in FIG. 2.

In FIG. 2, a curve shown by the solid line indicates longitudinal lightdistribution, and a curve shown by the dashed line indicates laterallight distribution. The term “longitudinal light distribution” means thedistribution of light emitted from the tip of the pen-type lamp, and theterm “lateral light distribution” means the distribution of lightemitted from the lateral side of the pen-type lamp. The angles indicatedalong a circular arc are emission angles of UV light emitted from thelamp placed at the center of the circular arc, and the distance from thecenter represents relative radiation intensity.

As can be seen from FIG. 2, the intensity distribution of UV lightemitted from the pen-type low-pressure mercury lamp is not uniform. Morespecifically, the relative intensity of UV light emitted from the lampplaced at the center of the circular arc measured at an emission angleof 30° or less is 90% or more, and the relative intensity of UV lightemitted from the lamp measured at an emission angle of 45° or more is80% or less.

Since the intensity of UV light attenuates as the distance between thepen-type low-pressure mercury lamp and an irradiated subject increases,the distance between the lamp and an irradiated subject is preferably assmall as possible. However, the area of a region irradiated withhigh-intensity UV light decreases as the distance between the lamp andan irradiated subject decreases. Therefore, the inventors of the presentinvention have studied the relationship between the flow channel patternof an irradiated subject and UV light intensity.

FIG. 3A is a plan view of an example of the oxidation channel 68 havingan unoptimized flow channel pattern, and FIG. 3B is a plan view of anexample of the oxidation channel 68 having an optimized flow channelpattern. In each of FIGS. 3A and 3B, a region surrounded by the dashedline indicates a region in which the pen-type low-pressure mercury lampis placed. In the case of the oxidation channel 68 shown in FIG. 3A, thechannel pattern complicatedly meanders from the water sample inlet 64 tothe through hole 70. In FIG. 3A, the diagonally shaded areas representregions in which the relative radiation intensity of UV light is 80% orless. On the other hand, in the case of the oxidation channel 68 shownin FIG. 3B, the channel pattern meanders from the water sample inlet 64to the through hole 70 along the mercury lamp so as to be parallel withthe mercury lamp. In FIG. 3B, the diagonally shaded area represents aregion in which the relative radiation intensity of UV light is 90% ormore.

As can be seen from FIGS. 3A and 3B, the intensity distribution of UVlight with which the water sample is irradiated is not uniform, andtherefore, there is a case where the concentration of carbon dioxidegenerated by oxidation in the water sample varies depending on the timeelapsed since the start of feeding of the water sample to the carbondioxide separation part.

FIGS. 4A and 4B show the results of conductivity detection using theoxidation channels shown in FIGS. 3A and 3B, respectively. The capacityof the oxidation channel shown in FIG. 3A is about 130 μL, and the watersample introduced through the water sample inlet 64 is fed to the watersample channel 2 at a flow rate of 100 μL/min after the completion ofoxidation of organic substances. In this case, part of the water sampleirradiated with UV light of which relative radiation intensity is 80% orless arrives at the water sample channel 2 after a lapse of about 40 to70 seconds. On the other hand, the capacity of the oxidation channelshown in FIG. 3B is about 200 μL, and a water sample introduced throughthe water sample inlet 64 is fed to the water sample channel 2 at a flowrate of 100 μL/min after the completion of oxidation of organicsubstances. In this case, part of the water sample irradiated with UVlight of which relative radiation intensity is 90% or more arrives atthe water sample channel 2 after a lapse of about 30 to 90 seconds.

As described above, FIG. 4A is a graph showing the result of experimentperformed using the instrument shown in FIG. 1 having the flow channelpattern shown in FIG. 3A, and FIG. 4B is a graph showing the result ofexperiment performed using the instrument shown in FIG. 1 having theflow channel pattern shown in FIG. 3B. In these experiments, an aqueouspotassium hydrogen phthalate solution (hereinafter, simply referred toas “KHP”) which is easily decomposed by oxidation and an aqueoustrimethylamine hydrochloride solution (hereinafter, simply referred toas “TMA”) which is not easily decomposed by oxidation were used as watersamples. Each of these two water samples had a concentration of 2.2mgC/L. The acquisition of conductivity was performed after a lapse ofabout 60 seconds since the start of feeding of the water sample. This isbecause the conductivity of KHP is preferably as stable as possible. Theacquired conductivity was converted into TOC using a calibration curve.

As can be seen from FIG. 4A, in the case of KHP, the conductivity isincreased for about 60 seconds after the start of feeding of KHP. On theother hand, in the case of TMA, the conductivity reaches its peak aftera lapse of about 40 seconds since the start of feeding of TMA, andtherefore, the conductivity measured after a lapse of about 60 secondssince the start of feeding of TMA is lower than its peak value.Therefore, in a case where the conductivity measured after a lapse of 60seconds since the start of feeding of TMA is converted to TOC, the TOCof TMA lower than its true value is obtained, thus resulting in anerror. It can be estimated that this is due to the following reasons:KHP can be decomposed even when the relative radiation intensity of UVlight with which KHP is irradiated is as low as 80% or less, whereas TMAis fed to the carbon dioxide separation part 20 without beingdecomposed.

On the other hand, the flow channel pattern shown in FIG. 3B is designedso that part of the water sample irradiated with UV light of whichrelative radiation intensity is 90% or more can be sufficiently fed tothe carbon dioxide separation part 20 after a lapse of 60 seconds sincethe start of feeding of the water sample. Therefore, as shown in FIG.4B, in both cases of KHP and TMA, the conductivity is not decreased evenafter a lapse of about 60 seconds since the start of feeding of thewater sample. In this case, the TOC of TMA is a true value.

Further, in the case of using the flow channel pattern shown in FIG. 3B,part of the water sample irradiated with UV light of which relativeradiation intensity is 90% or less arrives at the carbon dioxideseparation part 20 after a lapse of 0 second since the start of feedingof the water sample and flows through the carbon dioxide separation part20 for 30 seconds. This is necessary for the following reasons. When awater sample is transferred from the organic substance oxidation part tothe carbon dioxide separation part, carbon dioxide contained in thewater sample is diffused into the dead volume of the carbon dioxideseparation part so that the concentration of carbon dioxide attenuatesin the carbon dioxide separation part. This causes a phenomenon in whichthe concentration of carbon dioxide measured by the conductivitymeasuring part is lowered. Therefore, in order to prevent the loweringof the concentration of carbon dioxide measured by the detection unit asmuch as possible, a water sample of which organic substances aredecomposed by oxidation to some extent is fed to the carbon dioxideseparation part just before the detection of conductivity to increasethe concentration of carbon dioxide in the dead volume of the carbondioxide separation part. This makes it possible to prevent the loweringof detection sensitivity.

As has been described above, according to the present invention, thecontrol unit 40 stops the feeding of a water sample during oxidation oforganic substances and performs the feeding of the water sample onlywhen the water sample is moved to the organic substance oxidation partand the carbon dioxide separation part, and the data processing unit 42determines the concentration of total organic carbon based onconductivity measured when the water sample irradiated with UV light ofwhich relative intensity is equal to or higher than a certain value isflowing through the carbon dioxide separation part. Therefore, it ispossible to reduce the consumption of the water sample and accuratelymeasure the TOC content of the water sample.

Further, as described above, in order to reduce the amount of the watersample to be fed to the carbon dioxide separation part as much aspossible also at times other than the time of measurement, the deadvolume of the carbon dioxide separation part 20 should be reduced asmuch as possible. The dead volume of the carbon dioxide separation part20 is mainly due to a porous fluorine resin membrane (gas-permeablemembranes 8 and 10), and therefore can be reduced by decreasing thevolume of gas contained in the porous fluorine resin membrane as much aspossible.

One method for decreasing the volume of gas contained in a porousfluorine resin membrane is to reduce the thickness of the membrane.However, the porous fluorine resin membrane needs to have a certaindegree of thickness due to manufacturing reasons. Another method fordecreasing the volume of gas contained in a porous fluorine resinmembrane is to reduce the area of the porous fluorine resin membrane 8as much as possible. Since the intermediate water channel 4 and themeasurement water channel 6 are present under the porous fluorine resinmembrane 8, the area of the porous fluorine resin membrane can bereduced by minimizing the area of contact between the porous fluorineresin membrane and the water sample channel 2, the intermediate waterchannel 4, or the measurement water channel 6.

A smaller depth of a flow channel for use in separating carbon dioxidemakes the time required to transfer carbon dioxide by diffusion shorter(this is because according to a diffusion equation, the transfer time isproportional to the square of the distance) so that carbon dioxide istransferred into another pure water in a shorter time. This makes itpossible to reduce the length of the flow channel. That is, by reducingthe depth of a flow channel for use in separating carbon dioxide as muchas possible, it is possible to reduce the length of the flow channel,which leads to a reduction in the area of the porous fluorine resinmembrane.

FIG. 5 is an exploded perspective view of another embodiment of thetotal organic carbon measuring instrument according to the presentinvention. As in the case of the embodiment shown in FIG. 1, thisembodiment shown in FIG. 5 is formed by integrating the organicsubstance oxidation part, the carbon dioxide separation part, and theconductivity measuring part. The oxidation channel 68 is formed as agroove having a depth of 0.6 mm in a top surface 62 a of the quartzsubstrate 62, and the water sample channel 2 is formed as a groovehaving a thickness of 60 μm in a back surface 62 b of the quartzsubstrate 62. The intermediate water channel 4 is formed as a throughgroove in a 100 μm-thick membrane 98 a made of an adhesive fluorineresin (e.g., Neoflon EFEP (“Neoflon” is a trademark of Daikin IndustriesLtd.)), and the measurement water channel 6 is formed as a groove havinga depth of 60 μm in a surface 80 a of the quartz substrate 80 (seeFIG. 1) opposed to the gas-permeable membrane 10. The oxidation channel68, the water sample channel 2, the intermediate water channel 4, andthe measurement water channel 6 have a meandering shape to increasetheir flow channel lengths. These channels 2, 6, and 68 can be formedby, for example, sandblasting.

The conductivity measuring part has two electrode patterns. One of theelectrode patterns is provided on a surface of the substrate 80 oppositeto the surface 80 a at a position corresponding to a position indicatedby the reference numeral 76 a, and the other electrode pattern isprovided on a surface of the quartz substrate 74 (see FIG. 1) opposed tothe substrate 80 at a position corresponding to a position indicated bythe reference numeral 76 a. A flow channel through which the measurementwater flows is formed by cutting a flow channel pattern out of a 100μm-thick membrane 78 a made of an adhesive fluorine resin (e.g., NeoflonEFEP (“Neoflon” is a trademark of Daikin Industries Ltd.)) to beinterposed between the substrates 80 and 74.

A shielding membrane 14 having openings is provided between theintermediate water channel 4 and the water sample channel 2 to adjustthe area of contact between the water sample and the intermediate water.Further, a shielding membrane 16 having openings is provided between theintermediate water channel 4 and the measurement water channel 6 toadjust the area of contact between the intermediate water and themeasurement water. An example of the shielding membranes 14 and 16includes a 25 μm-thick membrane made of an adhesive fluorine resin(e.g., Neoflon EFEP ((“Neoflon” is a trademark of Daikin IndustriesLtd.)). An example of the gas-permeable membranes 8 and 10 includes a 30μm-thick porous fluorine resin member (e.g., Poreflon (“Poreflon” is atrademark of Daikin Industries Ltd.).

The total organic carbon measuring instrument shown in FIG. 5 is formedby laminating the above-described quartz substrate 62, gas-permeablemembrane 8, shielding membrane 14, adhesive fluorine resin membrane 98a, shielding membrane 16, gas-permeable membrane 10, quartz substrate 80a, electrode pattern 76 a, and adhesive fluorine resin membrane 78 a inthis order from top to bottom, sandwiching them between a substrate asan uppermost layer corresponding to the quartz substrate 60 shown inFIG. 1 and a substrate as a lowermost layer corresponding to the quartzsubstrate 74 shown in FIG. 1, and bonding them together for integration.

1. A total organic carbon measuring instrument comprising: a measuringunit having an organic substance oxidation part for oxidizing organicsubstances contained in a supplied water sample to carbon dioxide byirradiation with UV light, a carbon dioxide separation part having awater sample channel through which a water sample transferred from theorganic substance oxidation part flows and a measurement water channelthrough which measurement water constituted from deionized water flows,the carbon dioxide separation part being formed by integrating the watersample channel and the measurement water channel being in contact witheach other gas-permeably and laminated in this order from top to bottom,and a conductivity measuring part for measuring a conductivity of themeasurement water transferred from the carbon dioxide separation part,the measuring unit being formed by laminating the organic substanceoxidation part, the carbon dioxide separation part, and the conductivitymeasuring part in this order from top to bottom; a control unit forcontrolling the operation of liquid delivery in the measuring unit; anda data processing unit for determining a concentration of total organiccarbon from a conductivity of measurement water, wherein the controlunit stops feeding the water sample during oxidation of organicsubstances and performs feeding the water sample when the water sampleis moved to the carbon dioxide separation part through the organicsubstance oxidation part, and wherein the data processing unitdetermines the concentration of total organic carbon based on theconductivity measured when the water sample irradiated with UV light ofwhich relative intensity is equal to or higher than a certain value isflowing through the carbon dioxide separation unit.
 2. The total organiccarbon measuring instrument according to claim 1, wherein the carbondioxide separation part has, between the water sample channel and themeasurement water channel, an intermediate water channel in whichintermediate water having a pH higher than that of the water sampleflowing through the water sample channel but within a neutral range ispresent, and wherein the water sample channel and the intermediate waterchannel are in contact with each other with a gas-permeable membranebeing interposed therebetween, and the intermediate water channel andthe measurement water channel are in contact with each other withanother gas-permeable membrane being interposed therebetween, andwherein the water sample channel, the intermediate water channel, andthe measurement water channel are laminated in this order from top tobottom and integrated.
 3. The total organic carbon measuring instrumentaccording to claim 1, wherein part of the water sample channel being incontact with the gas-permeable membrane meanders to have an increasedflow channel length.
 4. The total organic carbon measuring instrumentaccording to claim 1, wherein part of the measurement water channelbeing in contact with the gas-permeable membrane meanders to have anincreased flow channel length.
 5. The total organic carbon measuringinstrument according to claim 2, wherein part of the intermediate waterchannel being in contact with the gas-permeable membranes meanders tohave an increased flow channel length.
 6. The total organic carbonmeasuring instrument according to claim 2, wherein a buffer solutionhaving a pH within a neutral range is used as the intermediate water. 7.The total organic carbon measuring instrument according to claim 3,wherein part of the measurement water channel being in contact with thegas-permeable membrane meanders to have an increased flow channellength.
 8. The total organic carbon measuring instrument according toclaim 2, wherein part of the water sample channel being in contact withthe gas-permeable membrane meanders to have an increased flow channellength, wherein part of the measurement water channel being in contactwith the gas-permeable membrane meanders to have an increased flowchannel length, and wherein part of the intermediate water channel beingin contact with the gas-permeable membranes meanders to have anincreased flow channel length.